I’ve emailed and gotten responses from Chaim Potok and Adam Curry; hand wrote Mitch Albom and got a canned response letter w/ a real signature.
I’ve sent condolence cards to the White House when a family member passes and always get a lovely thank you for it, no matter the administration.
I wrote to Star Trek novel author Diane Carey, & she sent me a nice, hand-written postcard.
When I was 13, I wrote a giddy two-page letter to Paul Simon (on whom I had an incandescent adolescent crush) but never got around to mailing it. I did keep it though, and it’s hilarious to look at it now.
This was back when George McGovern was running for president, and I had created my own slogan, which I used incessantly to greet/say goodbye to people: “Peace, Paul Simon power, and George McGovern for president!” So I of course I explained this in my letter, and I drew a picture of Snoopy holding a sign with my slogan written on it.
If I go back and read the letter now, when I finally stop laughing I am (a) overcome with relief that I didn’t embarrass myself by actually sending it; and (b) reminded of what a totally unbearable, thoroughly obnoxious 8th grader I was.
(I’ve been lurking here for years but never actually posted before today)
He’s not exactly my favorite celebrity, but I am Facebook friends with Charlie Sheen. And yeah, I’m reasonably certain it’s the genuine article. A friend of mine mentioned hanging out with him, and I sent Charlie a message asking if this happened. He responded very nicely and said he didn’t remember the guy in question, but he doesn’t remember a lot of things and could very well have met him before.
I wrote a letter to Terri Nunn, lead singer of Berlin, to ask her for an interview for my junior college newspaper and she wrote me back saying yes, and instructed me to contact her manager to arrange it. The manager pretty much blew me off, so I wrote her again, so she called my home to arrange it personally.
And she was kind and beautiful in person. 
I’ve written a few authors (science fiction and fantasy), and most have replied at least once. I don’t know if my success rate is due to me being a wonderful fan letter writer, or SF/fantasy writers knowing that they NEED fans, or a combination. But when I’ve received replies, they’ve always been very nice and personalized.
But some of them invite you for brunch 
When I was a teeny-bopper in the 60s, I wrote to a few of my musical idols, even inviting one group to stay at our house if they ever performed in Baltimore. (This is especially hilarious when you know that we had 7 people living in a 3 BR/1BA, 1000 sq ft row house.) I think one letter yielded a tour schedule, but that was it. After that, I quit writing to the rich and famous. I was crushed…
I wrote to Professor J. R.R. Tolkien, and got a reply.
Then I lost it. (sob)
I’m sorry for your loss.
Which was lovely–but I have been (politely) stalking Jean Dujardin and Robert Downey, Jr., for strictly business reasons for *months *and cannot get so much as a “fuck off” from them!
I sort of had a crush on a writer who wrote a daily poetical column about nature in an national newspaper. Most of those columns were about him being invited to some nature area and him telling what he saw there. I sent him a letter through the paper, inviting him to a guided tour in a forest near me with with fire flies (kinda special here in the Netherlands) .I got a handwritten postcard with no retrun adress saying thanks, but no thanks. I was thrilled and embarrassed at the same time. I thought part of his refusal was that I had come across as a fan instead of a colleague nature enthusiast.
Once I sent an encouraging email to my favourite national politician, Marianne Thiemeof the Animal Rights Party. At the time, they had not gotten enough votes to get a seat in parliament, but it had been very close. I told her how uninterested I had been in politics before she had established that party, and how much I had enjoyed campaigning for the party. And that she should go on.
I got a personal e-mail back saying that the many mails like mine she got, had prompted her to go on. I printed it, kissed it and put it in my scrap book. 
When I was 14 I wrote a fan letter to Isaac Asimov. He personally wrote me a (brief) friendly reply that he typed on a post card. I was in seventh heaven.
It seems like Isaac Asimov was a popular person to send fan mail to.Thanks for the post.
I emailed Bryan Cranston shortly before his star took off with Breaking Bad, asking for an autographed photo. I received one several months later. Later, I wrote back congratulating him on his Emmy win for Breaking Bad, and received a brief reply, which I believe was from him personally, thanking me and remarking on the similarity between my name and an upcoming character on the show.
When I was 13, I wrote a gushing letter to Dean Cain, star of Lois and Clark. A few weeks later I received a form postcard with a picture and a printed greeting and signature. I was simultaneously bummed and thrilled at the same time, but had it in my school locker for years.
And now he’s on some trashtastic Fox celebrity dating show, so serves him right for not marrying me when he had the chance.
A letter? Screw that. I wanted to call.
Did you know that Andrew Lloyd Weber is not in the Rochester, NY phone book?
I wrote to Tom Lehrer a couple of years ago after reading in an interview that his address “was in the phone book” and, after some resarch, finding a Thomas A Lehrer on the White Pages online. Never did get a reply, so either I had the wrong man, or perhaps he’d just decided he wasn’t interested in replying to fan mail any more. I just wanted to tell him how much I’d enjoyed his music over the years.
I’m going to start watching that show and looking for a character named needsdecaf.
Electronics nerds will know who Bob Pease is. (well, was) I emailed him a few times. He usually wrote back. Actually he replied to one of my emails several times…must of been some glitch that kept my original popping up in his in box. He published one of my missives in his column. Another time he sent me a big packet of info on the hoax that is the Taguchi method and other six-sigma nonsense.